1. Field of the Invention
This invention relates to burners for use in high temperature industrial processes, such as steel reheating, aluminum melting, glass melting, mineral processing and the like.
2. Description of the Related Art
Conventional high temperature industrial process, such as steel reheating, have furnace exhaust gas temperatures in excess of 2000.degree. F. Because these high exhaust temperatures result in decreased energy efficiency and increased fuel costs, recuperative and regenerative heat exchangers have been employed to recover the exhaust heat by preheating the combustion air. While high-performance burners of this type have the potential to increase fuel efficiency, they produce high peak gas temperatures which lead to high NO.sub.x emissions. This limits application of the burners in areas where air quality and No.sub.x emission regulations are enforced.
Heretofore, high air temperature preheat burners have been developed which employ certain NO.sub.x control techniques. A principal technique is to suppress peak flame temperatures by dilution of the combustion reactants with inert gases, such as through the use of external flue gas recirculation, and/or by heat transfer from the flame zone, such as through enhanced flame emissivity.
Low NO.sub.x burners for high air temperature preheat applications have been developed through minor modifications to existing burners, and these modified burners produce moderate NO.sub.x reductions. More aggressive designs have drastically lowered flame temperatures from the levels expected at high furnace and air preheat temperatures, but these designs compromise flame stability during furnace heat-up, when furnace and air preheat temperatures are low. To overcome these problems, burners have been employed which use mechanical valves to alter hot flue gas or air flows, or have even used supplemental burners to heat up the furnace. However, these approaches are costly and unreliable.
Among the prior burners employing aggressive NO.sub.x reduction designs for lowering NO.sub.x are those in which the number of air ports is reduced, the distance between the fuel jet and air jets is increased and the quarl depth is reduced. While in such designs NO.sub.x has been lowered to below 150 ppm at up to 750.degree. F. preheat, the disadvantages are that flame momentum and chamber recirculation are reduced, and there are significant gas temperature non-uniformities.
Another prior art aggressive burner design for reducing NO.sub.x is that in which secondary air is injected around the fuel jet, and a valve is used to control the air flows to the annular and outer air jets. Although in such a design the gas temperature non-uniformities can be controlled, the flame character improved and NO.sub.x emissions lowered, the complexity, unreliability and cost of controlling air injection makes this burner design undesirable.
Another prior burner design using an aggressive approach to NO.sub.x reduction is that in which a fixed air jet burner has its fuel injector recessed into the refractory quarl for purposes of inducing furnace gas recirculation to the fuel jet. In such a design NO.sub.x levels are below 150 ppm at up to 750.degree. F. preheat while maintaining a compact design and adequate flame momentum.
In yet another prior low-NO.sub.x burner design for air preheats up to 1200.degree. F., a conventional fuel injector and inner set of air ports are surrounded by outer, tertiary air ports formed in the refractory quarl. Air flow into the tertiary ports is controlled by mechanical valves. This increases the complexity and cost of the burner design. In operation of such a burner, NO.sub.x is approximately 150 ppm at 750.degree. F. air preheat, rising to approximately 200 ppm at 1200.degree. F. air preheat.
The industry has sought a burner design which will reduce NO.sub.x to very low levels at air preheats up to the highest level allowed by recuperator or regenerative heat exchangers. In addition to NO.sub.x emission control effectiveness, it is desirable that the burner maintain optimum heat transfer, and that combustion (i.e. low CO and unburned hydrocarbon emission levels) and stability characteristics be consistent with those achieved by high-performance burners. In addition, it is desirable that the burner design operate with acceptable costs, reliability and turn-down capability. The ability to retrofit the burner into existing combustion chambers is also desirable.